In MRI investigation of body, heart and chest, MRI data acquisition is limited by respiratory motion of the body to breath hold periods in order to reduce image degradation due to motion. To extend imaging time, methods of navigator echo have been implemented to measure the position of the diaphragm in multiple breath hold periods. Image acquisition is shifted to a new location as determined by the measured position of the diaphragm. A single line 1D FT image is obtained perpendicular to the diaphragm to measure the location of the diaphragm during breath holding. The level of the image acquisition is shifted in space by the measured displacement from the diaphragm's position in the first breath hold period of MRI data acquisition. Organs other than the heart, kidney, liver and pancreas have also been imaged with multiple breath hold using MRI navigator echoes. Several variants of multiple-breath hold with navigators exist but have achieved limited success due to inherent errors in measurements and variations in the correlation between measured position of the diaphragm and the heart.
The movement of an organ such as the heart during an MRI scan can and typically does cause artifacts in the image. The artifacts are due mostly to non-linearities in the data set before Fourier Transformation or back-projection reconstruction of the MRI image. Generally it is not possible to know where the moving organ was located during the MRI data acquisition without some form of gating. Gating is a process in which the MRI data collection is timed to a certain temporal point with respect to a repetitive trigger of the MRI pulse sequence. EKG signals are often used to trigger MRI pulse sequences such that each MRI signal in the final data set is obtained at the same time in the cardiac cycle and the same cardiac phase. Another approach to this problem is to oversample the data without cardiac gating so that an average position is obtained for each line of k-space in MRI signal processing. This oversampling method, taken alone, may reduce artifacts, but is still limited in obtainable spatial resolution to the time averaged position of the heart. A variant of oversampling called “retrospective navigator” (RNAV) is more effective in improving the spatial resolution in the image. The RNAV method in MRI essentially acquires an additional selective line image (1D FT line image) placed on the diaphragm or more directly on the heart or other organ to be imaged. For each line of k-space data, an RNAV signal is also obtained. The two sets of data are 1D FT processed independently and the RNAV line images are evaluated. The RNAV lines show the position of the organ and if the organ is within a certain acceptable position, the time of the RNAV is stored. The k-space data acquired at the stored time are accepted. RNAV times are selected for the final k-space data set (RNAV corrected k-space), discarding k-space data that were acquired during non-acceptable displacements. FT image reconstruction of the RNAV corrected data gives an MRI image without major blurring and with increased spatial resolution. There are limitations to the accuracy of the RNAV method, especially in the correlation between a 1D image and the 3D motion of the heart. The RNAV method works poorly when there is abnormal non-periodic motion in 3D space, as occurs with people with cardiac disease and respiratory disease that is perhaps secondary the their cardiac function.
Thus, MRI imaging and MRI pulse sequences can make use of “MRI navigator echoes” (NavEcho). The NavEcho is an MRI signal that is not directly used to make the image; instead the NavEcho signal is used to obtain information on the heart, diaphragm or other body organ that is used to improve the final image of that organ. The improvement is usually by means of correcting for body motion either by prospectively modifying an MRI pulse sequence or by retrospectively rejecting signals that have moved out of some boundary region. An example is to use a single line scan through the right diaphragm as the NavEcho. The image acquisition of the heart is then only permitted when the NavEcho identifies the diaphragm as being at a certain position. The heart position is correlated to the diaphragm position and this measure of diaphragm position effectively permits signal acquisitions of the heart only at times when the heart is in the same or nearly same position. The process reduces blurring and artifacts, up to the accuracy and reproducibility in the correlation of the positions of the two organs. Because the NavEcho signal is not acquired on the heart, there is no loss of heart MRI signal.
If the NavEcho signals are positioned on the heart, the subsequent image signals would experience a shortened T1 recovery time (time between the NavEcho and the excitation of the image signal). This would directly reduce the quality and resolution of the final image, causing signal loss and artifacts. The problem of NavEcho and signal interference would be worse using a 2D rather than a 1D NavEcho such as could be achieved with a sub-second EPI image. In this case, an entire plane of the EPI image located through the heart interferes with the NavEcho signal.
Using NavEcho requires precious time during which the image signal acquisition cannot be performed. The NavEcho and the image signals are competing for acquisition time. They are not acquired simultaneously. The ADC sampling is performed typically first for the NavEcho and then for the image signal. If a 3D image of the heart were used for the NavEcho, it would take a prohibitively long amount of time, e.g., an estimated 500 milliseconds (ms) to acquire data for a 3D GRASE single shot image. It might then take another 500 ms to perform the 3D FT and to extract an edge position on the heart. This information could be used only after a delay of 1 second or so before the information is available to direct the MRI image signal acquisition. This can be entirely useless since the heart would then typically be in an entirely different phase of a different cardiac cycle. Single line (1D) NavEcho requires only a few milliseconds (ms) to acquire, and a 1D Fourier Tranform (FT) can be processed and used within less than 100 ms. This time for NavEcho acquisition and processing of its information is useful to reposition image acquisition within the same cardiac cycle, although the temporal resolution of this process is fairly low given the heart's continuous movement in 3D space. It is not surprising that the NavEcho has limited usefulness in improving coronary MRA beyond the current 3 mm3 resolution. Yet a different problem with using NavEchoes is that people with diseased hearts typically do not have predictable or reproducible respiratory motion or cardiac motion and, consequently, the NavEcho methods have poor accuracy in predicting position and timing. Thus, the NavEcho works less well in the patient population for which a highly accurate NavEcho for coronary MRA is most desired.
The idea of image fusion has been proposed for functional MRI (fMRI) and magnetoencephalography (MEG) whereby information is taken from each and combined in an image display. Spatial distribution of fMRI information can give a map of where brain activation occurs but at very low temporal resolution, down to half a second temporal resolution in some experiments. The MEG instead has very high temporal resolution, less than 50 ms, but it has less well defined spatial localization. Digital image maps are often displayed in color and show information from the MEG and fMRI combined in a ‘fusion image’.
It would be desirable to find a way to guide MRI imaging prospectively, retrospectively, or both, or substantially in real time, in a way that would provide real time or near real time guidance, would not interfere with the MRI signal needed for imaging the organs of interest, and would be convenient to implement and use, but no known technique existed to meet those goals well.